1. Field of the Invention
The invention is generally directed at metered dose inhaler (MDI) formulations which utilize non-ozone depleting propellants. More specifically, the invention is directed to MDI formulations which include 1,1,1,2,3,3,3-heptafluoropropane (HFC-227) as a propellant.
2. Description of the Prior Art
There are two types of formulations administered using pressurized MDIs. In conventional solution-type MDIs, drug is dissolved with the aid of non-volatile co-solvents such as ethanol. Conversely, in suspension formulations, small micronized particles of undissolved drug are distributed in the propellant or propellant blend. When a patient actuates the valve, a precisely measured dose of a drug is released and subsequently inhaled. Large particles or droplets in the spray impact in the oropharynx. By contrast, smaller particles (1-10 .mu.m) are required for penetration into the bronchioles or pulmonary regions of the lung. It is therefore necessary that suspension-type MIDIs be formulated with "potentially respirable" micronized particles (median diameter of approximately 3 .mu.m) and that these particles do not grow during the shelf life of the product. Growth can lead to less penetration of drug into the lung and disrupt operation of the metering valve.
Surface active compounds or "surfactants" are used in MDI formulations to aid in the dissolution or suspension of the drug in the propellant or propellant blend. The surfactants also serve to improve valve function by virtue of their lubricating properties. In order to achieve these objectives however, the surfactant must be dissolved in sufficient concentrations. For example, surfactant should ordinarily be at approximately 0.01-5% weight in volume (w/v). Often, the surfactant is incorporated at about 1/10th the concentration of the drug in the MDI formulation.
Currently, chlorofluorocarbon (CFC) blends are used as propellants in MDIs. CFC-11, CFC-12, and CFC-114 are the most widely used propellants in MDI formulations. However, use of CFC substances has come under criticism in recent years because they are widely believed to be damaging to the Earth's ozone layer. The Montreal Protocol on Substances that Deplete the Ozone Layer is an international treaty that has been signed by most industrialized countries and it prescribes a gradual phase out of CFC substances by the end of 1995. The treaty restrictions are a difficult burden on the MDI industry since no suitable propellants have been identified as "drop-in" replacements for CFCs, in that they would require little or no modification to drug formulations, formulating techniques, and materials used in MDIs.
Two hydrofluorocarbon (HFC) gases, 1,1,1,2-tetrafluoroethane (134a) and 1,1,1,2,3,3,3-heptafluoropropane (227), are currently considered as the most viable CFC alternatives for use in MDIs. However, because these two excipients have not been assessed or approved by any government authority, they must undergo the same degree of toxicological testing which is required for any new drug substance. The International Pharmaceutical Aerosol Consortiums for Toxicology Testing (IPACT-I for 134a and IPACT-II for 227) have been organized to test the HFCs and compile a safety data package suitable for satisfying the leading health authorities around the world. Members of these consortia will be able to reference the compiled data package for each excipient. However, they will be required to perform bridging studies on their own reformulated MDI products.
The reformulation of MDIs with alternative propellants requires a variety of criteria to be met. First, the drug should be easily dissolved or dispersed within the propellant. Partial dissolution, however, can result in problems with crystal growth over time. Uniform distribution of the drug within the propellant assures that the drug dose administered per each actuation is constant. Second, the surfactant should dissolve within the propellant or propellant blend at the required concentration. Third, if a blend of propellants is used, the blend should be single phase at room temperature. Fourth, the particle size of the drug following spraying should duplicate the size patterns which are now available with CFCs so that the new formulations are at least as efficacious as those currently in use. Fifth, the MDI formulation (e.g., surfactant and propellant or propellant blend) should be compatible with the elastomer seals and valve components used in the MDI canister to prevent leakage which results from shrinking and to prevent valve jamming which results from swelling. Sixth, the MDI formulation should be physically and chemically stable for an extended period of time. Seventh, for suspension formulations, the drug should be readily dispersed after standing. Eighth, for suspension formulations, the suspension should remain homogenous for the period between shaking, firing, and releasing the valve so as to refill the metering chamber.